1. Field of the Invention
The invention relates to methods and apparatus for fabricating semiconductor devices, such as memory devices and logic devices.
2. Description of the Related Art
Processes for fabricating semiconductor devices generally require beginning with a silicon substrate and growing or depositing additional layers on the substrate. The various steps require placing a series of masks on the various layers to define the circuitry. By etching around the masks and implanting certain impurities, desired configurations and performance characteristics of circuits are achieved. NAND and NOR type flash memories are examples of semiconductor circuits with different characteristics.
Often a large number of steps and masks and complicated processes are required to construct desired circuits. A significant factor in the manufacturing process is the number of masks which must be used. The greater the number of masks, the greater the number of steps which must be performed. As a result, manufacturing time increases with the number of masks and the cost of the semiconductor device increases accordingly. As a result of market factors, purchasers view many semiconductor devices as commodities. Thus, purchasers of semiconductor devices are highly price sensitive. Reducing the manufacturing time and the number of steps in the manufacturing process is a significant factor in controlling manufacturing costs. The degree of process difficulty is also a factor impacting manufacturing costs. Ways of controlling manufacturing costs include reducing the number of masks and simplifying the processes required to manufacture a semiconductor device. In addition to reducing manufacturing time and lowering costs, reducing the number of masks and simplifying the processes reduce the defect level and errors produced during the manufacturing process. Thus using fewer masks in the manufacturing process tends to increase product yield.
In view of the factors discussed above, it is an object of the invention to reduce the number of steps and simplify the processes required in the manufacturing process of semiconductor devices.
It is still a further object of the invention to improve semiconductor yields.
It is a still further object of the invention to reduce the number of masks and simplify the processes required to fabricate a semiconductor device.
The above and other objects of the invention are accomplished with a high yield simple process to fabricate and manufacture semiconductor devices such as NAND type flash memory devices. In one aspect of the invention a NAND type flash memory device is fabricated using a process modified from processes used to fabricate NOR type memories. According to another aspect of the invention xe2x80x9ca reverse field oxide processxe2x80x9d, which reverses the sequence in which the core and peripheral areas of a semiconductor device are initially formed, avoids an initial mask step required in conventional processes. This approach also allows the subsequent alignment for well implants.
In another aspect, the invention also eliminates an n-well mask and a thick gate transistor Vt implant mask when optimizing the formation of thick oxides for high voltage and thinner oxides for low voltage transistors to provide separate threshold voltage control.
In further steps, the process according to the invention eliminates the need for a poly silicon cap, the need for a high voltage implant mask and the need for a middle dope drain (MDD) implant mask, thereby saving two masks in this portion of the process.
In another aspect of the invention, a P+ contact implant mask is eliminated further reducing the cost.